A conventional chimney is constructed of masonry units which may include brick, stone, or concrete block. A flue is the space or passage through which gaseous matter is vented in conjunction with fireplaces, furnaces, and boilers. It is constructed so as to eliminate fire hazards due to heat conduction and dissipation.
Most building regulations require a flue lining of some type, which is available in hollowed rectangular, square or round shapes in a vitrified finish. Clay tiles (liners) are two foot sections of burned clay, shale, fireclay, or mixtures thereof, designed to resist corrosion, softening or cracking from excessive flue gas temperatures. A flue lining within a chimney extends from approximately one foot below the flue connection to the top of the chimney. Tile-lined masonry chimneys have an air gap between the tile and brick (roughly 1/2"-1"), which allows the tile thermal expansion and contraction, without stressing the chimney wall.